Synthesis method of 2, 4-dichlorobenzaldehyde

Abstract

The invention discloses a synthesis method of 2, 4-dichlorobenzaldehyde, and belongs to the technical field of unsaturated halogenated hydrocarbons, the method comprises the following steps: diazotization, formylation, quenching and purification; the diazotization method comprises the following steps: dissolving 2, 4-dichloroaniline in an HCl solution, placing the solution in an ice salt bath, cooling to 0 DEG C, dropwise adding a NaNO2 solution through a constant-pressure funnel, controlling the dripping speed to be 1-2 drops per second, controlling the system temperature to be 0-2 DEG C, adding urea after dropwise adding is finished, and carrying out heat preservation and stirring for 25-35 minutes to form a diazonium salt suspension; the method for preparing the 2, 4-dichlorobenzaldehyde has the advantages of high yield, high purity and few byproducts, the total yield can reach 82.7-84.9%, the purity is 98.72-99.03%, and the content of the byproduct 3, 5-dichlorobenzaldehyde in a crude product is 1t; and 2%.

Description

Technical Field

[0001] The present invention belongs to the technical field of unsaturated halogenated hydrocarbons, and particularly relates to a method for synthesizing 2,4-dichlorobenzaldehyde. Background Art

[0002] As an important organic synthesis intermediate, 2,4-dichlorobenzaldehyde has shown extensive application value in multiple fields such as medicine, pesticides, and fragrances. In the pharmaceutical field, it is a key starting material for synthesizing various drugs with specific pharmacological activities, capable of constructing drug molecular skeletons with unique structures and functions; in the pesticide aspect, it can be used to prepare highly efficient and low-toxic pesticide varieties, playing an important role in ensuring the healthy growth of crops; in the fragrance industry, its special chemical structure endows it with unique aroma characteristics, and it can be used to formulate various fragrant spice products.

[0003] Currently, the main synthesis methods of 2,4-dichlorobenzaldehyde are as follows.

[0004] Direct oxidation method of 2,4-dichlorotoluene: This method uses 2,4-dichlorotoluene as the raw material and synthesizes 2,4-dichlorobenzaldehyde through an oxidation reaction. In actual operation, common oxidants include potassium permanganate, potassium dichromate, etc. However, this method has many disadvantages. On the one hand, a large amount of wastewater containing heavy metal ions is generated during the reaction. If these wastewaters are directly discharged without proper treatment, they will cause serious pollution to the environment and bring great pressure to the ecological environment; on the other hand, the reaction selectivity is poor, and it is easy to generate various by-products, such as 2,4-dichlorobenzoic acid, etc. This not only reduces the yield of the target product 2,4-dichlorobenzaldehyde, but also increases the difficulty of subsequent separation and purification, resulting in a significant increase in production costs.

[0005] Chlorination and hydrolysis method of 2,4-dichlorotoluene: This method uses 2,4-dichlorotoluene as the raw material. First, side-chain chlorination is carried out under light to generate 2,4-dichlorobenzal dichloride, and then the target product is obtained by hydrolysis under the action of a catalyst. This reaction requires high-temperature and high-pressure conditions, with high energy consumption, low safety, and a low total product yield and many by-products.

[0006] In summary, the existing synthesis methods of 2,4-dichlorobenzaldehyde have the disadvantages of complex co-produced components and large separation difficulty, and there are many deficiencies in terms of environmental protection, safety, and yield. Therefore, it is of great practical significance and application value to develop a synthesis method of 2,4-dichlorobenzaldehyde with simple steps, safe operation, high yield, high purity, and few impurities. Summary of the Invention

[0007] Aiming at the deficiencies of the existing technology, the present invention provides a method for synthesizing 2,4-dichlorobenzaldehyde, which has simple steps, safe operation, high yield, high purity, few impurities, and easily available raw materials.

[0008] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for synthesizing 2,4-dichlorobenzaldehyde, comprising the following steps: diazotization, formylation, quenching and purification; The method of diazotization is as follows: Dissolve 2,4-dichloroaniline in an HCl solution, place it in an ice-salt bath and cool it to 0 °C. Dropwise add the NaNO2 solution through a constant pressure funnel, control the dropping speed at 1-2 drops / second, control the system temperature at 0-2 °C. After the dropping is completed, add urea, keep warm and stir for 25-35 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 5.5-6.5 mol / L; The concentration of the NaNO2 solution is 38-40 g / L; The mass ratio of 2,4-dichloroaniline, HCl solution, NaNO2 solution, and urea is 50:160-180:81-85:9-10.

[0009] The method of formylation is as follows: Premix N,N-dimethylformamide and anhydrous ZnCl2, control the temperature in an ice bath at 0-2 °C, dropwise add POCl3 under stirring at a speed of 200-300 rpm, control the dropping speed at 20-40 drops / minute. After the dropping is completed, keep warm for 15-20 minutes to obtain a formylation reagent. Cool the formylation reagent to -5 °C to 0 °C, cool the diazonium salt suspension to 0-2 °C, under stirring at a speed of 250-350 rpm, dropwise add the diazonium salt suspension to the formylation reagent, control the dropping speed at 25-35 drops / minute, control the system temperature at 0-5 °C. After the dropping is completed, raise the temperature to 20-25 °C, stir and react in the dark for 40-50 minutes to obtain a crude product solution; The mass ratio of N,N-dimethylformamide, anhydrous ZnCl2, and POCl3 is 80:24-28:50-60; The mass ratio of the diazonium salt suspension to the formylation reagent is 300-325:154-168.

[0010] The method of quenching and purification is as follows: Slowly pour the crude product solution into a mixture of crushed ice and water, while stirring at a speed of 400-500 rpm, adjust the pH to 6.5-7.0 with a NaOH solution, then add ethyl acetate for extraction in 3-4 batches to obtain an organic phase. Combine the organic phases and wash with a saturated NaHCO3 solution to remove the residual POCl3, then dry with anhydrous magnesium sulfate. The filtrate after filtration is distilled under reduced pressure to recover the solvent to obtain a crude product. Then dissolve the crude product in hot ethanol at 58-62 °C, cool to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde; The mass ratio of crushed ice to water in the mixture of crushed ice and water is 10:5-8; The concentration of the NaOH solution is 30-40 wt%; The mass ratio of the crude product solution, ice - water mixture, and all ethyl acetate is 100:130 - 150:220 - 240; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:10 - 15; The cooling temperature for cooling and precipitating crystals is -8°C to -10°C; The mass ratio of the crude product to hot ethanol is 40:80 - 100.

[0011] In the diazotization step, hydrochloric acid reacts with sodium nitrite to form nitrous acid (HNO2), and nitrous acid is protonated in strong acid to form an electrophilic reagent NO + , and the amino nitrogen atom of 2,4 - dichloroaniline undergoes nucleophilic attack on NO + , forming an N - nitroso intermediate (Ar - NH - N = O), and through proton transfer and dehydration, a diazonium salt (Ar - N2 + ) is formed; In the diazotization step, adding urea can decompose the excess nitrous acid to avoid oxidation side reactions.

[0012] In the formylation step, POCl3 reacts with DMF to form a chloroimine salt, which forms a super - electrophile through coordination with ZnCl2. In the presence of a strong electrophilic reagent, the diazonium salt undergoes synchronous denitrogenation - electrophilic addition. After the diazonium salt denitrogenates, the electron - withdrawing effect of the 2,4 - dichloro substituent stabilizes the aryl cation intermediate and inhibits side reactions; in the electrophilic substitution reaction, although the electron density at the 6 - position (para to 2 - Cl) is slightly higher, the low temperature below 5°C inhibits the electrophilic substitution path, and the rate constant of the aryl cation path is much higher than that of the electrophilic substitution; The electrophilic substitution at the 6 - position (para to 2 - Cl) with slightly higher electron density will produce a by - product 3,5 - dichlorobenzaldehyde.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention prepares 2,4 - dichlorobenzaldehyde through a diazotization - formylation tandem reaction. This method only requires two steps, with simple steps, a concise route, and the raw material 2,4 - dichloroaniline being low - cost and easily available.

[0014] (2) In the diazotization step of the present invention, by strictly controlling the temperature and dropping rate, the diazonium salt exists in a precipitate suspension state, reducing hydrolysis side reactions and improving the utilization rate of the diazonium salt; the added urea avoids nitrous acid oxidation by - products and ensures the purity of the diazonium salt; in the present invention, the conversion rate of the suspended diazonium salt > 95%.

[0015] (3) In the formylation step of the present invention, ZnCl2 is used in combination with POCl3 / DMF to form a super - electrophile, and the enhanced electrophilicity significantly increases the formylation rate of the diazonium salt, and the target aldehyde yield can reach 80 - 85%.

[0016] (4) The whole process of the present invention requires a low temperature, consumes less energy, has no violent heat release / gas surges, has a low operating risk coefficient, and high safety.

[0017] (5) The method for preparing 2,4-dichlorobenzaldehyde of the present invention has the advantages of high yield, high purity, and few by-products. The total yield can reach 82.7 - 84.9%, the purity is 98.72 - 99.03%, and the content of by-product 3,5-dichlorobenzaldehyde in the crude product is <2%. Specific Embodiments

[0018] In order to have a clearer understanding of the technical features, objectives, and effects of the present invention, the specific embodiments of the present invention will now be described.

[0019] Example 1 A method for synthesizing 2,4-dichlorobenzaldehyde, comprising the following steps: (1) Diazotization Dissolve 50 g of 2,4-dichloroaniline in 160 g of HCl solution, cool it to 0 °C in an ice-salt bath, add 81 g of NaNO2 solution dropwise through a constant pressure funnel, control the dropping rate at 1 drop / second, control the system temperature at 0 °C, add 9 g of urea after the dropping is completed, and keep stirring at a constant temperature for 35 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 6.5 mol / L; The concentration of the NaNO2 solution is 38 g / L.

[0020] (2) Formylation Premix 80 g of N,N-dimethylformamide with 24 g of anhydrous ZnCl2, control the temperature at 0 °C in an ice bath, add 50 g of POCl3 dropwise with stirring at 200 rpm, control the dropping rate at 20 drops / minute, keep stirring at a constant temperature for 15 minutes after the dropping is completed to obtain a formylation reagent, cool the formylation reagent to -5 °C, cool the diazonium salt suspension to 0 °C, and drop the diazonium salt suspension into the formylation reagent with stirring at 250 rpm, control the dropping rate at 25 drops / minute, control the system temperature at 0 °C, and raise the temperature to 20 °C after the dropping is completed, and stir the reaction in the dark for 50 minutes to obtain a crude product solution.

[0021] (3) Quenching and Purification The crude product solution was slowly poured into a crushed ice - water mixture while stirring at a speed of 400 rpm. The pH was adjusted to 6.5 with NaOH solution, and then ethyl acetate was added in 3 batches for extraction to obtain the organic phase. The combined organic phase was washed with saturated NaHCO3 solution to remove residual POCl3, dried with anhydrous magnesium sulfate, and the solvent in the filtered filtrate was recovered by distillation under reduced pressure to obtain the crude product. Then the crude product was dissolved in hot ethanol at 58 °C and cooled to -8 °C to precipitate crystals, obtaining the finished product of 2,4 - dichlorobenzaldehyde; The mass ratio of crushed ice to water in the crushed ice - water mixture is 10:5; The concentration of the NaOH solution is 30 wt%; The mass ratio of the crude product solution, the crushed ice - water mixture to all of the ethyl acetate is 100:130:220; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:10; The mass ratio of the crude product to the hot ethanol is 40:100.

[0022] The content of 2,4 - dichlorobenzaldehyde in the crude product prepared in this example is 86.45%, and the content of the by - product 3,5 - dichlorobenzaldehyde is 1.54%; This example used 50 g of 2,4 - dichloroaniline and finally obtained 45.46 g of the finished product of 2,4 - dichlorobenzaldehyde with a purity of 98.72% and the total yield could reach 83.1%.

[0023] Example 2 A method for synthesizing 2,4 - dichlorobenzaldehyde, comprising the following steps: (1) Diazotization 50 g of 2,4 - dichloroaniline was dissolved in 170 g of HCl solution, cooled to 0 °C in an ice - salt bath, and an 83 g NaNO2 solution was added dropwise through a constant - pressure funnel at a dropping rate of 1.5 drops / second, controlling the system temperature at 1 °C. After the addition, 9.5 g of urea was added, and the mixture was stirred while maintaining the temperature for 30 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 6 mol / L; The concentration of the NaNO2 solution is 39 g / L.

[0024] (2) Formylation Premix 80 g of N,N-dimethylformamide with 26 g of anhydrous ZnCl2, control the temperature at 1 °C in an ice bath, add 55 g of POCl3 dropwise with stirring at 250 rpm, control the dropping rate at 30 drops per minute, keep warm for 18 minutes after the addition is completed to obtain a formylation reagent. Cool the formylation reagent to -2 °C and cool the diazonium salt suspension to 1 °C. Stir at 300 rpm and add the diazonium salt suspension dropwise to the formylation reagent, control the dropping rate at 30 drops per minute, control the system temperature at 2 °C, and heat to 22 °C after the addition is completed. Stir the reaction in the dark for 45 minutes to obtain a crude product solution.

[0025] (3)Quenching and purification Slowly pour the crude product solution into a mixture of crushed ice and water, stir at 450 rpm at the same time, adjust the pH to 6.8 with NaOH solution, then add ethyl acetate for extraction in 3 batches to obtain an organic phase. Combine the organic phases and wash with saturated NaHCO3 solution to remove residual POCl3, then dry with anhydrous magnesium sulfate. Distill the filtrate under reduced pressure to recover the solvent to obtain a crude product. Then dissolve the crude product in hot ethanol at 60 °C and cool to -9 °C to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde; The mass ratio of crushed ice to water in the mixture of crushed ice and water is 10:6; The concentration of the NaOH solution is 35 wt%; The mass ratio of the crude product solution, the mixture of crushed ice and water to all ethyl acetate is 100:140:230; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:12; The mass ratio of the crude product to hot ethanol is 40:90.

[0026] The content of 2,4-dichlorobenzaldehyde in the crude product prepared in this example is 86.80%, and the content of the by-product 3,5-dichlorobenzaldehyde is 1.41%; This example uses 50 g of 2,4-dichloroaniline, and finally obtains 46.30 g of the finished product of 2,4-dichlorobenzaldehyde with a purity of 99.03% and a total yield of 84.9%.

[0027] Example 3 A method for synthesizing 2,4-dichlorobenzaldehyde, comprising the following steps: (1)Diazotization Dissolve 50 g of 2,4-dichloroaniline in 180 g of HCl solution, cool to 0 °C in an ice-salt bath, add 85 g of NaNO2 solution dropwise through a constant pressure funnel, control the dropping rate at 2 drops per second, control the system temperature at 2 °C, add 10 g of urea after the addition is completed, and keep warm and stir for 25 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 5.5 mol / L; The concentration of the NaNO2 solution is 40 g / L.

[0028] (2) Formylation Premix 80 g of N,N-dimethylformamide with 28 g of anhydrous ZnCl2, control the temperature at 2 °C in an ice bath, add 60 g of POCl3 dropwise with stirring at 300 rpm, control the dropping rate at 40 drops per minute, keep warm for 20 minutes after the addition is completed to obtain a formylation reagent. Cool the formylation reagent to 0 °C and cool the diazonium salt suspension to 2 °C. With stirring at 350 rpm, drop the diazonium salt suspension into the formylation reagent, control the dropping rate at 35 drops per minute, control the system temperature at 5 °C, and after the addition is completed, raise the temperature to 25 °C and stir the reaction in the dark for 40 minutes to obtain a crude product solution.

[0029] (3) Quenching and purification Slowly pour the crude product solution into a crushed ice-water mixture while stirring at 500 rpm, adjust the pH to 7 with NaOH solution, then add ethyl acetate for extraction in 4 batches to obtain an organic phase. Combine the organic phases, wash with saturated NaHCO3 solution to remove residual POCl3, then dry with anhydrous magnesium sulfate, and distill the filtrate under reduced pressure to recover the solvent to obtain a crude product. Then dissolve the crude product in hot ethanol at 62 °C and cool to -10 °C to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde; The mass ratio of crushed ice to water in the crushed ice-water mixture is 10:8; The concentration of the NaOH solution is 40 wt%; The mass ratio of the crude product solution, the crushed ice-water mixture and all ethyl acetate is 100:150:240; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:15; The mass ratio of the crude product to hot ethanol is 40:100.

[0030] The content of 2,4-dichlorobenzaldehyde in the crude product prepared in this example is 85.91%, and the content of the by-product 3,5-dichlorobenzaldehyde is 1.58%; This example uses 50 g of 2,4-dichloroaniline, and finally obtains 45.17 g of the finished product of 2,4-dichlorobenzaldehyde with a purity of 98.89%, and the total yield can reach 82.7%.

[0031] Example 4 A method for synthesizing 2,4-dichlorobenzaldehyde, comprising the following steps: (1) Diazotization Dissolve 50 g of 2,4-dichloroaniline in 170 g of HCl solution, place it in an ice-salt bath and cool it to 0 °C. Dropwise add 83 g of NaNO2 solution through a constant pressure funnel, control the dropping rate at 1.5 drops per second, control the system temperature at 1 °C. After the dropping is completed, add 9.5 g of urea, keep warm and stir for 30 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 6 mol / L; The concentration of the NaNO2 solution is 39 g / L.

[0032] (2) Formylation Under stirring at 1 °C and 250 rpm, dropwise add 55 g of POCl3 to 80 g of N,N-dimethylformamide, control the dropping rate at 30 drops per minute. After the dropping is completed, keep warm for 18 minutes to obtain a formylation reagent. Cool the formylation reagent to -2 °C, cool the diazonium salt suspension to 1 °C. Under stirring at 300 rpm, dropwise add the diazonium salt suspension to the formylation reagent, control the dropping rate at 30 drops per minute, control the system temperature at 2 °C. After the dropping is completed, raise the temperature to 22 °C, stir the reaction in the dark for 45 minutes to obtain a crude product solution.

[0033] (3) Quenching and purification Slowly pour the crude product solution into a mixture of crushed ice and water, and stir at 450 rpm at the same time. Adjust the pH to 6.8 with NaOH solution, then add ethyl acetate for extraction in 3 batches to obtain an organic phase. Combine the organic phases and wash them with saturated NaHCO3 solution to remove the residual POCl3, then dry them with anhydrous magnesium sulfate. The filtrate after filtration is distilled under reduced pressure to recover the solvent to obtain a crude product. Then dissolve the crude product in hot ethanol at 60 °C and cool it to -9 °C to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde; The mass ratio of crushed ice to water in the mixture of crushed ice and water is 10:6; The concentration of the NaOH solution is 35 wt%; The mass ratio of the crude product solution, the mixture of crushed ice and water to all ethyl acetate is 100:140:230; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:12; The mass ratio of the crude product to the hot ethanol is 40:90.

[0034] The content of 2,4-dichlorobenzaldehyde in the crude product prepared in this example is 34.82%, and the content of the by-product 3,5-dichlorobenzaldehyde is 3.57%; This example uses 50 g of 2,4-dichloroaniline, and finally obtains 15.21 g of the finished product of 2,4-dichlorobenzaldehyde, with a purity of 96.24% and a total yield of 27.1%.

[0035] Example 4 In the formylation step, POCl3 was added dropwise to N,N-dimethylformamide to prepare the formylation reagent. Without adding anhydrous ZnCl2, the content of 2,4-dichlorobenzaldehyde in the crude product was low, and the final yield was less than 30%. It can be seen that without the synergistic effect of ZnCl2, the formylation reaction rate of the formylation reagent is low, and the formylation reaction cannot be completed according to the formylation reaction time of the present invention, indicating that the hyper-electrophile formed by the synergistic effect of ZnCl2 and POCl3 / DMF can significantly improve the formylation reaction rate.

[0036] Example 5 A method for synthesizing 2,4-dichlorobenzaldehyde, comprising the following steps: (1) Diazotization 50 g of 2,4-dichloroaniline was dissolved in 170 g of HCl solution, cooled to 0 °C in an ice-salt bath, 83 g of NaNO2 solution was added dropwise through a constant pressure funnel, the dropping rate was controlled at 1.5 drops / second, the system temperature was controlled at 1 °C, and after the dropping was completed, 9.5 g of urea was added, and the mixture was stirred at a constant temperature for 30 minutes to form a diazonium salt suspension; The concentration of the HCl solution is 6 mol / L; The concentration of the NaNO2 solution is 39 g / L.

[0037] (2) Formylation Under stirring at 1 °C and 250 rpm, 55 g of POCl3 was added dropwise to 80 g of N,N-dimethylformamide, the dropping rate was controlled at 30 drops / minute, and after the dropping was completed, the mixture was kept warm for 18 minutes to obtain the formylation reagent. The formylation reagent was cooled to -2 °C, and the diazonium salt suspension was cooled to 1 °C. Under stirring at 300 rpm, the diazonium salt suspension was added dropwise to the formylation reagent, the dropping rate was controlled at 30 drops / minute, the system temperature was controlled at 2 °C, and after the dropping was completed, the temperature was raised to 22 °C, and the reaction was stirred in the dark for 180 minutes to obtain a crude product solution.

[0038] (3) Quenching and purification The crude product solution was slowly poured into a crushed ice-water mixture, and at the same time stirred at 450 rpm. The pH was adjusted to 6.8 with NaOH solution, and then extracted with ethyl acetate in 3 batches to obtain an organic phase. The organic phases were combined and washed with saturated NaHCO3 solution to remove residual POCl3, then dried with anhydrous magnesium sulfate, and the solvent was recovered by reduced pressure distillation of the filtrate after filtration to obtain a crude product. Then the crude product was dissolved in hot ethanol at 60 °C and cooled to -9 °C to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde; The mass ratio of crushed ice to water in the crushed ice-water mixture is 10:6; The concentration of the NaOH solution is 35 wt%; The mass ratio of the crude product solution, the crushed ice-water mixture and all ethyl acetate is 100:140:230; The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:12; The mass ratio of the crude product to hot ethanol is 40:90.

[0039] The content of 2,4-dichlorobenzaldehyde in the crude product prepared in this example is 76.84%, and the content of the by-product 3,5-dichlorobenzaldehyde is 5.37%; This example uses 50 g of 2,4-dichloroaniline, and finally obtains 40.28 g of 2,4-dichlorobenzaldehyde finished product with a purity of 96.59% and a total yield of up to 71.4%.

[0040] Example 5 is based on Example 4, and the stirring reaction time in the formylation reaction is increased to 180 minutes. Compared with Example 4, the total yield increases, but the by-product content of the crude product also increases, and the purity and yield of the finally obtained finished product are both lower than those of Examples 1-3.

[0041] Unless otherwise specified, the percentages described in this application are all mass percentages.

[0042] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for synthesizing 2,4-dichlorobenzaldehyde, characterized in that, The synthesis method includes the following steps: diazotization, formylation, quenching and purification; The method of diazotization is as follows: dissolve 2,4-dichloroaniline in HCl solution, place it in an ice-salt bath and cool it to 0 °C. Dropwise add the NaNO2 solution through a constant-pressure funnel, control the dropping rate at 1-2 drops / second, control the system temperature at 0-2 °C. After the dropping is completed, add urea and stir while keeping warm for 25-35 minutes to form a diazonium salt suspension; The method of formylation is as follows: premix N,N-dimethylformamide and anhydrous ZnCl2, control the temperature in an ice bath at 0-2 °C, dropwise add POCl3 while stirring at a speed of 200-300 rpm, control the dropping rate at 20-40 drops / minute. After the dropping is completed, keep warm for 15-20 minutes to obtain a formylation reagent. Cool the formylation reagent to -5 °C to 0 °C, cool the diazonium salt suspension to 0-2 °C. While stirring at a speed of 250-350 rpm, dropwise add the diazonium salt suspension into the formylation reagent, control the dropping rate at 25-35 drops / minute, control the system temperature at 0-5 °C. After the dropping is completed, raise the temperature to 20-25 °C and stir the reaction in the dark for 40-50 minutes to obtain a crude product solution.

2. The synthesis method of 2,4-dichlorobenzaldehyde according to claim 1, characterized in that, In the step of diazotization, The concentration of the HCl solution is 5.5-6.5 mol / L; The concentration of the NaNO2 solution is 38-40 g / L; The mass ratio of 2,4-dichloroaniline, HCl solution, NaNO2 solution, and urea is 50:160-180:81-85:9-10.

3. A method for synthesizing 2,4-dichlorobenzaldehyde according to claim 1, characterized in that, In the step of formylation, The mass ratio of N,N-dimethylformamide, anhydrous ZnCl2, and POCl3 is 80:24-28:50-60.

4. A method for synthesizing 2,4-dichlorobenzaldehyde according to claim 1, characterized in that, In the step of formylation, The mass ratio of the diazonium salt suspension to the formylation reagent is 300-325:154-168.

5. A method for synthesizing 2,4-dichlorobenzaldehyde according to claim 1, characterized in that, The method of quenching and purification is as follows: slowly pour the crude product solution into a crushed ice-water mixture while stirring at a speed of 400-500 rpm, adjust the pH to 6.5-7.0 with NaOH solution, then extract with ethyl acetate in 3-4 batches to obtain an organic phase. Combine the organic phases and wash with saturated NaHCO3 solution to remove the residual POCl3, then dry with anhydrous magnesium sulfate. The filtrate after filtration is distilled under reduced pressure to recover the solvent to obtain a crude product. Then dissolve the crude product in hot ethanol at 58-62 °C, cool to precipitate crystals to obtain the finished product of 2,4-dichlorobenzaldehyde.

6. The synthesis method of 2,4-dichlorobenzaldehyde according to claim 5, characterized in that, In the step of quenching and purification, The mass ratio of crushed ice to water in the crushed ice-water mixture is 10:5-8; The concentration of the NaOH solution is 30-40 wt%; The mass ratio of the crude product solution, crushed ice-water mixture, and all ethyl acetate is 100:130-150:220-240.

7. A method for synthesizing 2,4-dichlorobenzaldehyde according to claim 5, characterized in that, In the step of quenching and purification, The mass ratio of the organic phase to the saturated NaHCO3 solution is 100:10-15; The cooling temperature for precipitating crystals is -8 °C to -10 °C; The mass ratio of the crude product to the hot ethanol is 40:80-100.